1. Field of the Invention
The present invention relates to processes for surface treatment, of an insulating substrate, e.g.; by layer growth, deposition or etching methods, and more particularly of an insulating substrate subjected to a flux due to ion and electron bombardment from a plasma.
More specifically, the present invention relates to treatment processes in which the substrate is supported by an electrode, while the plasma is created independently of the electrode supporting the substrate.
2. Prior Art
In order to obtain a surface treatment with well-determined characteristics, it is important to control, among other things, the ion and electron flux bombarding the substrate.
A number of prior art processes exist for controlling the bombarding flux, and are adapted to regulate the biasing of the electrode placed in a plasma.
One such process, disclosed in Journal of Physics D: Appl. Phys. 19 (1986), pp 795-809 consists in applying, through a low impedance capacitor, a symmetrical sinusoidal or square-wave signal to a substrate placed on an electrode immersed in a continuous plasma.
In the above process, the low impedance capacitor connected in series with the voltage generator and the electrode ensures that the charges (positive ions and then electrons) are accumulated over one period, and hence provides self-regulation of the electrode biasing with respect to the plasma potential.
A first drawback with this process concerns the application of a sinusoidal signal when the frequency f of the applied signal is less than, or on the order of, the ionic plasma frequency fp.sub.+, that is: EQU .omega.&lt;.omega..sub.p+ (.omega.=2.pi.f),
with ##EQU1## where: .sub.p+ is the ionic plasma pulse,
n.sub.+ is the positive ion density in the plasma, PA1 m.sub.+ is the positive ion mass, PA1 .sub.o is the permittivity in vacuuo. PA1 e is the absolute electron charge. PA1 maintaining within the chamber a continuous plasma that is free of electromagnetic fields; PA1 supplying said electrode via low impedance capacitor using signal comprised of rectangular voltage pulses having:
In the above case, the energy dispersion of the ions bombarding the electrode is equal to the product of the electron charge e and the peak applied signal voltage. Although non-damaging for some surface treatments, this dispersion cannot be considered so for all surface treatments.
Apart from the energy distribution of the ions accelerated towards the electrode in the case of a low-frequency self-biaising (.omega.&lt;.omega..sub.p+), the prior art process cannot ensure a good control of the time distribution of the charged particles that bombard the electrode, namely the positive ions and electrons.
Japanese patent JP-6277 479 of Sept. 30, 1985 teaches the implementation of a plasma process in which an asymmetrical voltage waveform is applied, through a capacitor, across the two exciter electrodes for a capacitative discharge, in view of reducing the bombarding energy of the ions on the electrode supporting the substrate to be treated. This process clearly has a first drawback in that it is impossible to control the relative periods of ionic and electron bombarding without simultaneously modifying the bombarding energy of the positive ions on the two electrodes. This drawback arises from the fact that plasma excitation cannot be disassociated from the interaction of the same plasma with the electrodes. In other words, in that type of plasma, not only will the overall balance of the discharges in fact depend on all the variables that are external to be discharged, but also the interaction in fact, depend on all the variables that are external to be discharged, but also the interaction of the plasma with the electrodes.
The process also has a second drawback insofar as the sustaining of the discharge between the electrodes requires the presence of a sufficiently high voltage between the electrodes. Accordingly, there is generally required a bombarding energy on the order of, or in excess of, 150 eV to sustain the discharge, which prohibits the adjustment of the relative bombarding periods in the ionic bombarding energy ranges that are below the value corresponding to the breakdown and discharge sustaining voltage. In many instances e.g. of engraving, this process makes it difficult, if not impossible, to obtain a good degree of selectivity, which can only be achieved at a low energy, in other words in the absence of sputtering.
Finally, a lowering of the applied voltage, i.e. of the power injected into the plasma, introduces a corresponding drop in the plasma density and hence in the speed of etching.
Further, the surface chemical reactions that occur in the plasma are of an electrochemical nature, so that the time distribution of the charged particles bombarding the substrate constitutes a parameter that needs to be taken into consideration in the control of the etching process and, more generally, in surface treatments.